Oh thanks for breaking it down for me. When it comes to web development, for all intents and purposes, they can be thought of as one in the same since the browser version issued with a device is tied to OS version[1]. Of course some users will upgrade OS and/or browser, independently in some cases, which complicates things further.
In that link he mentions it but it bears a little breaking down. His chart is showing the versions of the rendering engine that is used to back a WebView. It isn't for the browser included with the phone. Google has requirements how WebViews are implemented but the requirements for the default browser are pretty light. Even if the user doesn't swap out their browser you don't know what you might get. See section 3.4 of the CDD for the requirements.
[1] http://jimbergman.net/webkit-version-in-android-version/